Battery draining

Hello folks,
I am coming across a weird electrical problem and really appreciate any advice or if someone have come across this before.
I changed the battery to a new one last year and has been working fine. A few months later I added a set of fog light and a roof light bar with their own dedicated switches and again everything has been working.
I also changed the bulb on the instrumentation panel as a few of them were brunt out and lastly the rear wiper motor was gone so replaced that too. Last weekend when I took my 90, 300TDI to work the battery went completely flat. With the help of couple of lads who gave me a push started it and brought it back home. Need to say that I don't drive it every day, once every couple of weeks and most of the time is sitting on the driveway putting up with the rain.
After taking the battery out and fully charging it (12.6 v on multimeter), started the engine and measured the voltage across battery terminal. Expected to see 14.4v but was reading only 12.2v.
Tested for any short circuit by disconnecting the negative clamp from the battery and measuring the Amps between the negative terminal and the clamp and get zero amps so guessing that have no short circuit. Rightly or wrongly I concluded that the alternator has had its days. A week later with a brand new 65A alternator fitted also drive belt replaced (even though the old was was good and not extended), I still get the same readings on the multi-meter. Tried a different Multi-Meter and again the same reading. Disconnecting the alternator wires completely and starting the engine on battery, measure the voltage on the alternator (between the big stud for the thick brown cable and the alternator grounded body) and I only get 10.0v.
Have I bought a faulty alternator? Is there an issue with my engine wiring loom? Anyone have come across this before?
Really appreciate any advice.
Thanks,
Mo

I think Halfords do a free battery check , when charged up it should be higher than 12.6
Is alternator earthed properly

Thank you Steve for your reply. The battery is relatively new and I have a digital charger that check the state of health of the battery also shows the % charge so I am confident is not the battery. Alternator is brand new from BEARMARCH (AMR4249), so I am confident that that is fine too.
Someone suggested in the conversation that it might be the voltage regulator which I don't know where is it and what it looks like so I am going to go and find out about that.

Post some pics of the rear of the alt - so we can see the wiring - it sounds like the alt is DOA, which does happen the parts lottery has dealt you a duff one, probably ....

Voltage decreasing when not running is probably a diode in the rectifier gone open circuit, rather than the regulator which should only affect charging voltage.....both are internal to the alternator...and are available on the bay for very little money if you've the confidence to change them...

For complete units, I use this lot - no affiliation, just a satisfied customer.

www.startermotor-alternator.co.uk

Mo younesi said:

wires completely and starting the engine on battery, measure the voltage on the alternator (between the big stud for the thick brown cable and the alternator grounded body) and I only get 10.0v.

Click to expand...

try the voltage check but earth to battery rather than alt body , I had charging probs and alt earth via fixings was not good

Here is the photos of both old and new alt connected. I only have the wires for the exciter and charging and don't have the rev counter.

Hello folks,
It has taken me a couple of weekends to get here, but still haven't got to the bottom of the problem.
Have now replaced the positive lead from the battery to the starter also have replaced the negative cable from the battery to the chassis and engine block making sure all contacts are grinned and clean, but still get only 12.2v when engine running across the battery terminal. I have measured from the positive stud on the alternator to the alternator body but reads only 12.2v. So I am almost now convinced that either the diodes in the brand new alternator are gone or the voltage regulator is gone.
I wonder if anyone could kindly advise on the following. The white wire that usually gets connected to the rev counter of the alternator was not connected to the old alternator so months ago I traced it back though the engine loom and cut it, hence with the new alternator as seen in the pictures the white wire is missing. Would this effect the performance of the voltage regulator, should I connect it back ?

Hopefully these fault finders might help

Many thanks Steve.
Good news is that the ebay seller is sending me a brand new replacement alternator which should arrive in a couple of days time and am optimistic that once I put the new one on I will see that magic 14.4V number.
The fault finding files will also be very handy for future issue so thank you for that.
Regards,
Mo

Result! Let us know if it solves the problem

Hello folks,
With many thanks again to all who contributed to this thread and advised, just to close off this thread also leave it such that it will be useful to anyone with similar problem:
The second new alternator replacing the first one arrived and was mounted. The result was the same and still 12.2 V at battery terminal with engine running, so I started going though all the cables:

• Replaced the Battery Positive to the starter,
• Battery negative to Chassis and the engine body (which is altogether a single cable) was taken off, cleaned and contact points to the chassis and engine were ground and cleaned
• Measured the voltage drop across the Thick brown cable from Alternator to the starter via the engine loom and the cable was good

Knowing that battery was good and all cables checked out, the only other element in the entire circuit was back to the voltage regulator, even though the alternator was new, so as it cost only a few pounds, bought a new voltage regulator and replaced. The end results was still the same 12.2 v.

this time round, i focused my search on the exciter wire (Yellow and brown color connected to D+ on the alternator). But firstly took the dash out and replaced the Battery warning light bulb with a new bulb to ensure it is not the bulb as the bulb works as a resistor in the exciter circuit.

• Turning the ignition and even running the engine, the warning light remained on, so I knew I am closing into the actual problem.
• Took the wrapping off the engine lume just before the bulkhead in the engine compartment and traced the yellow and brown exciter wire. Connect a temporary wire to this and then connected the other end to D+. Disconnected the yellow and brown from the D+. in effect replaced the exictor wire temporarily without having to ripe it out of the engine lume.
• Turn the ignition on and ******RESULTS*****, 14.6 V at last.
• I don't think I have ever been so F**king happy seeing a 14.6 V on the multimeter.
• All I have to do now is to tidy up the engine lume (Of course the Bonnet cable stopped working in the process of all above, so had to take the bonnet latch out and give it a good clean also replacing the cable. Lastly cut out that horrible little bracket that mounts the cable handle and making a new one strong one to rivet it to the bulk head, but that is another story.).

Cheers.

Forgot to mention that the Isolation switch was rubbish and don't think is intended for repeated use as the "L" shaped red key for it rides over a plastic cam profile when you turn the key on. This will wear out easily which will then you won't get a good solid contact inside the switch and will have arc and spark inside the switch.
I put it on about a year ago not only to stop any possible battery draining, but also as a security measure, so whenever I turned the engine off, I was taking the key out.
Most importantly I should not have put the isolation switch on the negative line. It should always be put on the positive line.
I have taken the damn thing out altogether and thrown into the bin, so here is an update to the earlier diagram in case someone may refer to it:

Well done thanks for the feedback